Printing applications using water-based ink include ink jet, flexography, gravure and screen-printing. The ink compositions used in these methods contain a water-soluble dye dissolved in water or in a combination of water with an organic solvent or a dispersed pigment.
Inks may be colored using either dyes or pigments. It is generally pointed out that an image obtained when using an ink containing a water soluble dye has poor water fastness and light fastness. In an image obtained when using an ink containing a pigment as a colorant, however, the colorant is likely to be left near the surface of the recording medium. An image obtained by ink containing pigment as a colorant has light fastness. This unsatisfactory fixation of the colorant on the surface of the recording medium results in the formation of a print in which, when the image is rubbed, the recording medium becomes smeared with the pigment. Prior art methods propose the addition of a resin to the ink composition in order to improve the fixation of the colorant onto the recording medium. The resin functions as a binder to strongly fix the colorant onto the recording medium. For example, laid-open Japanese Patent Application No. 07-157668 demonstrates a conventional resin-containing ink composition including, for example, an ink comprising a colorant dispersed in a water-insoluble, resin emulsion dispersion.
An ink jet recording method is a printing method wherein droplets of an ink composition are ejected and deposited onto a recording medium. This printing method has a feature that an image having high resolution and high quality can be realized at high speed with a relatively inexpensive apparatus.
U.S. Pat. No. 4,381,185 discloses a simple process to obtain a water-fast print on paper using a colorant solution containing water-soluble polymeric dyes. This is accomplished by using a paper employing substantial cation content, especially a substantial polyvalent metal ion content for example, Fe2+, Fe3+, Cu2+, Zn2+, Al3+, Mg2+, Ca2+ and Ba2+, and applying a solution containing one or more polymeric colorants possessing anionic net charge, for example, anionic groups.
Another ink jet recording method has been proposed in Laid-Open Japanese Patent Application No. 05-202328. This method comprises applying a polyvalent metal salt solution onto a recording medium and then applying an ink composition containing a dye having at least one carboxyl group. According to this method, polyvalent metal ions combine with the dye to form a precipitate, which can provide a high quality image having water resistance, and which is free from color bleeding.
Recording methods that comprise the steps of printing two solutions, a first solution and an ink composition, such as JP Application 05-202328 have also been disclosed, by way of example, in U.S. Pat. No. 5,948,512. A first solution contains a polyvalent metal salt and/or a polyallylamine, which is used in combination with an ink composition containing an inorganic oxide colloid, and optionally an epoxy-containing compound. The first solution is deposited onto a recording medium, and the ink composition is then deposited by ink jet printing, realizing an evenly printed image with no significant feathering, which is free from color bleeding.
Further, U.S. Pat. No. 6,084,619 presents an ink jet recording method in which a reaction solution and an ink composition are used to print on a recording medium. The reaction solution comprises a polyvalent metal salt, and the ink composition comprises a pigment and a resin emulsion. This method is capable of providing an image having good quality.
Another example of printing with two solutions is disclosed in U.S. Pat. No. 6,086,197, which presents an ink jet recording method including the step of depositing a reaction solution and an ink composition onto a recording medium. The ink composition used has a colorant, a resin emulsion and the like and the reaction solution has a reactant reactive with the colorant and/or the resin emulsion to produce an agglomerate, such as a polyvalent metal salt or a polyalylamine. The reaction solution and the ink composition each has a surface tension of less than 40 mN/m, reached by an addition of an anionic surfactant.
Yet another example is, U.S. Pat. No. 6,080,229, which discloses a reaction solution for use in a two solution ink jet recording method. The reaction solution comprises at least a polyvalent metallic salt and a component selected from the group consisting of five or six-member saturated or unsaturated, heterocyclic compound containing at least one nitrogen atom, an optionally alkyl-substituted amide derivative, a sulfur-containing compound, an amine, and a cationic surfactant. It possesses improved storage stability, can improve printing stability, and in addition, can offer a printed image having improved quality.
The two-solution printing method as described in the above patents, which uses a reaction solution and an ink composition, gives a good solution to the printing quality achieved on porous material such as papers and a fair solution to mechanical stability. However, the image mechanical stability, e.g., abrasion resistance and scratch resistance, on non/low-porous materials such as vinyl, anodized metal, various types of coated papers etc., is very poor. The prior art patents do not deal with this problem. When reaction solution is deposited on a non/low-porous material and then rapidly followed by the deposition of the ink composition, the two solutions react and precipitate all ink solids to form an ink dot. However, this precipitate ink dot has no interaction with the non/low-porous surface, as the reaction solution tends to form a liquid boundary between the ink and the recording media's surface. This mechanism damages the mechanical stability of the image, making it easy to rub off.
Recording media are generally not thermally stable and when exposed to high temperatures undergo a change in color. Prior art methods which use the two-solution printing method further compromise the thermal stability of the recording media, i.e., a recording media which has been treated with reaction solution and is subjected to a drying process tends to change its color at a significantly lower temperature than untreated media. The media turns slightly reddish to brown-black, depending on the dryer temperature and the polyvalent metal comprised in the reaction solution.
It would be desirable to provide a new surface treatment for printing applications using water-based ink, which would provide a quality printed image with increased mechanical and chemical stability and which would provide thermal stability so as to preserve true color of the recording media.